None.
The present invention relates to the field of direct material deposition processes which allow complex structures to be fabricated efficiently in small lots to meet stringent requirements of a rapidly changing manufacturing environment. More particularly, the invention petains to the fabrication of three-dimensional metal parts directly from a computer-aided design (CAD) electronic xe2x80x9csolidxe2x80x9d model. The invention also provides methods which use existing industry-standard computer file formats to create unique material structures including those having thermal characteristics embedded within them. The invention addresses methods to control direct material deposition processes to achieve a net-shaped or near net-shaped article, and to fabricate metal articles having exceptional material properties and dimensional repeatability.
Manufacturing techniques or technologies generally known as xe2x80x9clayered manufacturingxe2x80x9d have emerged over the last decade. For metals, the usual shaping process forms a part by removing metal from a solid bar or ingot until the final shape is achieved. With the new technique, parts are made by building them up on a layer-by-layer basis. This is essentially the reverse of conventional machining. According to the paper appearing at the Internet site of Helsinki University of Technology, the first commercial process was presented in 1987. The process then was very inaccurate, and the choice of materials was limited. The parts were considered, therefore, prototypes and the process was called rapid prototyping technology (RPT). The prior art has advanced, however, to a point where it has been favorably compared too conventionally numerically controlled (NC) milling techniques. Considerable savings in time, and therefore cost, have been achieved over conventional machining methods. Moreover, there is a potential for making very complex parts of either solid, hollow or latticed construction.
Stereolithography technique (SLT), sometimes known as solid freeform fabrication (SFF), is one example of several techniques used to fabricate three-dimensional objects. This process is described in the Helsinki University of Technology paper. A support platform, capable of moving up and down is located at a distance below the surface of a liquid photo polymer. The distance is equal to the thickness of a first layer of a part to be fabricated. A laser is focused on the surface of the liquid and scanned over the surface following the contours of a slice taken through a model of the part. When exposed to the laser beam, the photo polymer solidifies or is cured. The platform is moved downwards the distance of another slice thickness and a subsequent layer is produced analogously. The steps are repeated until the layers, which bind to each other, form the desired object. A Hexe2x80x94Cd laser may be used to cure the liquid polymer. The paper also describes a process of xe2x80x9cselective laser sintering.xe2x80x9d Instead of a liquid polymer, powders of different materials are spread over a platform by a roller. A laser sinters selected areas causing the particles to melt and solidify. In sintering, there are two phase transitions, unlike the liquid polymer technique in which the material undergoes but one phase transition: from solid to liquid and again to solid. Materials used in this process included plastics, wax metals and coated ceramics. A number of Patents and other disclosures have preceded and followed these processes, including the following:
U.S. Pat. No. 4,323,756, issued on Apr. 6, 1982 to Clyde O. Brown, et al., entitled Method for Fabricating Articles by Sequential Layer Deposition, discloses a method for the production of bulk rapidly solidified metallic objects of near-net shape, by depositing multiple thin layers of feedstock using an energy beam to fuse each layer onto a substrate. The feedstock may be in the form of metal powder or wire. A net shaped or near-net shaped article is one which approximates all of the desired features of its contemplated design so that little or no finishing work is required.
In his U.S. Pat. No.4,724,299, dated Feb. 9, 1988, Albert W. Hammeke describes a laser spray nozzle in which a beam passageway between the end portions permits a laser beam to pass through. A housing surrounds a second end portion and forms an annular passage, coaxial with the beam passageway. A cladding powder supply system is connected with the annular passage so that the powder exits the coaxial opening with the beam. The laser beam melts the powder which is deposited on a target substrate. The powder distribution system is contained within the nozzle assembly.
A laser spray nozzle assembly is a part of the Axial Flow Laser Plasma Spraying apparatus disclosed by Eric J. Whitney et al. in their August 1991 U.S. Pat. No. 5,043,548. The apparatus for depositing a feed material onto a substrate, has a plasma confinement chamber into which a laser beam is focused, the focal point being at a distance sufficiently far from the substrate that the substrate, is not melted. Finely divided feed material in a carrier gas flow is fed axially into the confinement chamber along the direction of the laser beam and melted into the plasma formed in the interaction of the laser beam, the feed material and the gas at the focal point. The feed material is then directed to deposit onto the substrate while the plasma energy is largely confined within the apparatus by the confinement chamber and constriction of the flow path upstream of the chamber.
A Rapid Prototyping System is disclosed by Joshua E. Rabinovich in U.S. Pat. No. 5,578,227, issued Nov. 26, 1996. The system involves a model making method and apparatus which projects a laser beam, circular polarizes the beam and directs the circular polarized beam for fusing a rectangular wire to a substrate or a previously fused wire on a target stage. The disclosure is differentiated by fusing the deposited feedstock to bond to a previously deposited layer without substantially altering the cross-section of the newly deposited material.
Such a deposition process would seem to have substantial problems of warping and distorting the deposited layers because of incomplete melting of feedstock material. Unlike Rabinovich""s disclosed process, a powder deposition completely consumes the feedstock material in the three-dimensional net shape. The powder""s cross-section and material properties are significantly altered. Rabinovitch does not disclose how the properties of the deposited material are controlled in his invention.
U.S. Pat. No. 5,697,046, dated Dec. 9, 1997 and entitled Composite Cermet Articles and Method of making was issued to Edward V. Conley. It discloses methods for making and using and articles comprising ferromagnetic cermets, preferably carbides and more preferably tungsten carbide having at least two regions exhibiting at least one property that differs. The cermets are manufactured by juxtaposing and densifying at least two powder blends having different properties. The methods described are very specific to cermets and do not employ solid models and automated processes.
U.S. Pat. No. 5,705,117 dated Jan. 6, 1998 discloses a Method of Combining Metal and Ceramic Inserts Into Stereolithography Components. Kurt Francis O""Connor et al. describe a stereolithography process for developing a prototype part in which inserts of non-photo polymer material are included in the resulting part so as to develop a functioning prototype part. In order to allow the inserts to be placed within the developing prototype part, a series of STL files are defined for forming the part in individual sections. The method is very specific to metal-ceramic composite structures for PC boards. It is not a direct fabrication method for three-dimensional objects with graded or multiple material structures.
Direct fabrication of three-dimensional metal parts by irradiating a thin layer of metal powder mixture is described in U.S. Pat. No. 5,393,613, entitled Composition for Three-Dimensional Metal fabrication Using a Laser, and issued Feb. 28, 1995. Colin A. MacKay uses a temperature equalization and unification vehicle in the mixture which is melted by a laser, selectively applied to form a solid metal film. The vehicle protects the molten metal from oxidation. The metal powder can contain an elemental metal or several metals. The material has a lower melting temperature because of the vehicle, which is essentially a flux. The method does not create structures of gradient material.
U.S. Pat. No. 5,707,715, issued to L. Pierre deRochemont et al. on Jan. 13, 1998, presents a disclosure of metal-ceramic composite comprising a metal member bonded to a ceramic oxide member through a covalent bond formed at temperatures less than 880 degrees Centigrade. Metal-ceramic composites are also described that are so constructed to control internal stress or increase crack resistance within the ceramic member under applied thermal or mechanical loads. The disclosure does not reveal a direct fabrication method for three-dimensional objects with graded or multiple material structures.
U.S. Pat. No. 5,126,102, entitled Fabricating Method of Composite Material, was granted to Masashi Takahashi on Jun. 30, 1992, and describes a method of preparing a composite material, excellent in joint strength and heat conductivity. More specifically, it describes a method of preparing a composite material composed of high melting temperature tungsten (W) material and low melting temperature copper (Cu) material by forming pores in the tungsten to obtain a substrate with distributed porosity. The method forms a high-porosity surface in at least one region of the substrate, the porosity gradually decreasing outward from the region. A second step impregnates the tungsten material with the copper material in the porous surface forming a gradient material of tungsten and copper. The patent describes the advantages of gradient materials, however, it does not discuss the use of solid models to achieve the shape of the gradient article. Direct material deposition processes produce three-dimensional parts by sequential layer deposition of feedstock material in powder or wire form.
Robert A. Sterett et al., in their aptly named U.S. Pat. No. 5,746,844, issued on May 5, 1998, disclose a Method and Apparatus for Creating a Free-Form Three-Dimensional Article Using A Layer-By-Layer Deposition of Molten Metal and Using Stress-Reducing Annealing Process On the Deposited Metal. A supply of substantially uniform droplets of desired material having a positive or negative charge, is focused into a narrow stream through an alignment means which repels each droplet toward an axis through the alignment means. The droplets are deposited in a predetermined pattern at a predetermined rate onto a target to form the three-dimensional article without use of a mold of the shape of the article. The disclosure reveals means for reducing stress by annealing portions of the deposited droplets which newly form a surface of the 3-D article. Melting of the metal is not done by laser and molten metal. Metal powder is carried from a liquid supply to the target surface. The invention produces xe2x80x9cfully densexe2x80x9d article of one metal or an alloy material having uniform density, no voids and no porosity. The method allows creation of part overhangs without using supports, by relying on the surface tension properties of the deposition metal.
U.S. Pat. No. 5,837,960 to Gary K. Lewis, of Los Alamos National Laboratory, et al. was filed on Nov. 30, 1995 and issued on Nov. 17, 1998. Its title is Laser Production of Articles from Powders. A method and apparatus are disclosed for forming articles from materials in particulate form in which the materials are melted by a laser beam and deposited at points along a tool path to form an article of desired shape and dimensions. Preferably, the tool path and other parameters of the deposition process are established using computer-aided design (CAD) and computer-aided manufacturing (CAM) techniques. A controller consisting of a digital computer directs movement of a deposition zone along the tool path and provides control signals to adjust the apparatus functions, such as the speed at which a deposition head which delivers the laser beam and powder to the deposition zone moves along the tool path. The article is designed using a commercially available CAD program to create a design file. A xe2x80x9ccutter location filexe2x80x9d (CL) is created from the design file and an adapted, commercially available CAM program. User-defined functions are established for creating object features in the adapted CAM program. The functions are created by passing an xe2x80x9celectronic planexe2x80x9d through the object feature. A planar figure created in the first plane at the intersection with the feature is a first portion of the tool path. A second plane is passed through the feature parallel to the first plane. The second plane defines a second tool path. The end of the tool path in the first plane is joined to the beginning of the tool path in the second plane by a movement command. The process is continued until the tool path required to make the feature is complete.
Lewis et al. describe certain methods of preheating an article support (substrate) to overcome the fact that without it, an article support will be cold when the deposition is started in comparison to the material on which deposition is later done in the fabrication process. Computer modeling of heat flow into, through and out of an article and the data generated from such modeling imported into the CAM program is suggested. The fabrication of articles of two different materials is addressed by forming a joint between dissimilar metals by changing powder compositions as the joint is fabricated. As an example, one could introduce a third material as an interlayer between mild steel and 304 stainless steel. The interlayer material might be a Nixe2x80x94Crxe2x80x94Mo alloy such as Hastelloy S.
U.S. Pat. No. 5,993,554 to David M. Keicher et al., dated Nov. 30, 1999 and entitled Multiple Beams and Nozzles to Increase Deposition Rate, describes an apparatus and method to exploit desirable material and process characteristics provided by a lower power laser material deposition system. The invention overcomes the lower material deposition rate imposed by the same process. An application of the invention is direct fabrication of functional, solid objects from a CAD solid model. A software interpreter electronically slices the CAD model into thin horizontal layers that are subsequently used to drive the deposition apparatus. A single laser beam outlines the features of the solid object and a series of equally spaced laser beams quickly fill the featureless regions. Using a lower power laser provides the ability to create a part that is very accurate, with material properties that meet or exceed that of a conventionally processed and annealed specimen of similar composition. At the same time, using multiple laser beams to fill in featureless areas allows the fabrication process time to be significantly reduced.
In an article entitled The Direct Metal Deposition of H13 Tool Steel for 3-D Components by J. Mazumder et al., the authors state that the rapid prototyping process has reached the stage of rapid manufacturing via direct metal deposition (DMD) technique. Further, the DMD process is capable of producing three-dimensional components from many of the commercial alloys of choice. H13 is a material of choice for the tool and die industry. The paper reviews the state of the art of DMD and describes the microstructure and mechanical properties of H13 alloy deposited by DMD.
The problem of providing a method and apparatus for optimum control of fabrication of articles having a fully dense, complex shape, made from gradient or compound materials from a CAD solid model, is a major challenge to the manufacturing industry. Creating complex objects with desirable material properties, cheaply, accurately and rapidly has been a continuing problem for designers. Producing such objects in high-strength stainless steel and nickel-based super alloys, tool steels, copper and titanium has been even more difficult and costly. Having the ability to use qualified materials with significantly increased strength and ductility will provide manufacturers with exciting opportunities. Solving these problems would constitute a major technological advance and would satisfy a long felt need in commercial manufacturing.
The present invention pertains generally to a class of material deposition processes that use a laser to heat and, subsequently, fuse powder materials into solid layers. Since these layers can be deposited in sequential fashion to ultimately form a solid object, the ability to alter the material properties in a very localized fashion has far reaching implications.
The present invention comprises apparatus and method for fabrication of metallic hardware with exceptional material properties and good dimensional repeatability. The invention provides a method for controlling material composition, and thus material characteristics, within a structure made from a plurality of materials, directly from computer renderings of solid models of the desired component. Both industry-accepted stereolithography (STL) file format as well as solid model file format are usable.
One embodiment of the invention is used to form embedded features in a three-dimensional structure. A plurality of separate material feedstock are fed into a directed material deposition (DMD) process which places a line of molten material onto a substrate. The depositions are repeated in a layer-by-layer pattern, defined by solid models which describe the structure, to create an article having complex geometric details. The bulk properties of the deposition are controlled by adjusting the ratio of laser irradiance to laser velocity along the line of deposition.
In addition to external contours, the solid-model computer files describe regions of each separate material, regions of a composite of the materials and regions of voids in each layer or xe2x80x9cslice.xe2x80x9d The depositions are repeated in each of the xe2x80x9cslicesxe2x80x9d of the solid models to create the geometric details within the three-dimensional structure.
Heating the substrate and the deposition produces parts with accurate dimensions by eliminating warping of the substrate and deposition. A prescribed temperature profile is used for processing tempered material. A temperature profile for heat treating may be used to enhance the mechanical properties of the part by ensuring the correct material microstructure during processing.
Although the prior use of DMD processes has produced solid structures, the use of this technology to embed features for thermal management of solid structures is novel. Embedding voids and/or composite material regions, enables thermal management engineering techniques for solid structures that are not available through conventional processing techniques. In one embodiment of the present invention, a method is provided to construct a solid structure with integral means to control its thermal properties.
Active thermal control is provided by forming passages and chambers for a coolant medium. The cross-section area and length of individual embedded structures are made approximately equal to provide uniform flow characteristics and pressure in the three-dimensional structure. Passive thermal control is provided by embedding materials having diverse thermal indexes.
Another embodiment of the present invention provides methods to locally control the thermal history of a three dimensional structure. Thermal history is the temperature variation in the part as a function of time. A part made with high thermal conductivity material in one region and a low thermal conductivity material in another region, will have a different thermal variation with time in each region.
In a further embodiment of the present invention, high-efficiency heat transfer is obtained within a three dimensional structure by incorporating regions of other materials within the article. For example, in parts having varying cross-sections, heating and cooling in selected regions is controlled to prevent thermal stresses.
In yet another embodiment of the present invention, three dimensional components are formed in which thermal characteristics such as heating and cooling rates are engineered into the component.
Embedding multi-material structures within a normally solid component, produces articles with diverse mechanical properties. Articles having complex internal and external contours such as heat exchangers and turbine blades are easily produced with the methods and apparatus disclosed.
To enhance the deposition process for manufacture of three-dimensional, multi-material structures with interior cavities either hollow or fixed with diverse material new apparatus, methods of deposition and material delivery are disclosed. These include:
1. Engineering properties such as tensile strength, toughness, ductility, etc. into the material layers by reference to a laser-exposure factor (E) which includes variables of laser power (p), relative velocity of the deposition (v) and material constants (a).
2. A fast-acting diverter valve for regulating feedstock flow allows precision depositions of gradient materials. The diverter valve controls the flow of a stream of a carrier gas and powder material to the deposition head. The valve comprises one diverter for a stream of gas only and another for a stream of gas and powder. The diverters are proportionately controlled so that the total volumetric flow rate of the powder and gas is constant, but the mass flow rate of powder to the deposition head can be quickly varied from no powder to the maximum available. Waste gas with powder is re-circulated and waste gas is reclaimed.
3. A self-contained, volumetric, low-friction powder feed unit which allows a user to use extremely low flow rates with a variety of powder materials; the powder feeder design is a marked improvement over current disk-style powder feeders in which the disk typically is buried in powder. In the present invention, powder flow from a reservoir to a transfer chamber is limited by the angle of repose of the powder feedstock, preventing the disk from being overwhelmed and clogged with powder. The present invention is insensitive to variations in flow rate of the gas which transports the powder to the deposition head. The spacing between the feed disk and the wipers which remove powder from the disk can be greater than in prior art designs without losing control of powder metering. This promotes much less wear on the wipers and substantially improves the life of the powder feed unit.
4. A multi-axis deposition head, including the powder delivery system and optical fiber, laser beam delivery system, moveable about a plurality of translational and rotational axes; the relative directions of the powder stream in the deposition process (123) being coordinated with a control computer (129) in a plurality of coordinate axes (x, y, z, u, v).
5. xe2x80x9cSmartxe2x80x9d substrates which are useful for construction of articles with internal spaces, unreachable from the surface, but serve as a starting point for conventional shaping methods.
6. Protection for the fiber optic which delivers a laser beam to the work piece to prevent catastrophic failure of the fiber because of beam reflections from the deposition surface, using a folding mirror, offset from 45 degrees by a small angle, to image a reflected laser beam at a distance from the fiber optic face, and water cooling of the fiber optic face.
7. A laser beam shutter with a liquid-cooled beam xe2x80x9cdumpxe2x80x9d to aid testing and adjustment of the fiber optic, laser beam delivery system.
8. Using the surface tension property of melted materials to creating structures having unsupported overhanging edges.
9. Using a rotated plane of deposition or rotating a multi-axis deposition head to build unsupported overhanging edges.
10. Particle beam focusing to reduce material waste.
An appreciation of other aims and objectives of the present invention may be achieved by studying the following description of preferred and alternate embodiments, and by referring to the accompanying drawings.